Postcards, letters, large envelopes (flats) and small packages can be sent using First-Class Mail®. This service, along with Priority Mail and Express Mail, is ideal for sending personal correspondence, handwritten or typewritten letters, and bills or statements of account. It may also be used for advertisements and lightweight merchandise. First-Class Mail prices are based on both the shape and weight of the item being mailed. For items weighing more than 13 ounces, use Priority Mail.